Pulp, slurry, sheet, laminate, and method for producing pulp

ABSTRACT

An object is to optimize a bleaching process in a step of producing phosphorylated cellulose fibers. A pulp comprises cellulose fibers having 0.5 mmol/g or more of phosphoric acid groups or phosphoric acid group-derived substituents, when the pulp is processed into a sheet and four sheets are laminated on one another, the ISO whiteness of the laminate is 82% or more. Moreover, when the pulp is processed into a sheet and four sheets are then laminated on one another, the b* value of the obtained laminate according to the L*a*b* color system is 5.5 or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 16/338,255 filed Mar. 29, 2019, which is a National Stage ofInternational Application No. PCT/JP2017/035525 filed Sep. 29, 2017,claiming priority based on Japanese Patent Application No. 2016-193393filed Sep. 30, 2016, all of which are incorporated herein by referencein their entirety.

TECHNICAL FIELD

The present invention relates to a pulp, a slurry, a sheet, a laminate,and a method for producing a pulp. Specifically, the present inventionrelates to pulp having high whiteness, slurry having low yellowness whenit is formed in a sheet, a sheet having low yellowness, and a laminatehaving the aforementioned sheet, and a method for producing theaforementioned pulp.

BACKGROUND ART

Conventionally, cellulose fibers have been broadly utilized in clothes,absorbent articles, paper products, and the like. As cellulose fibers,ultrafine cellulose fibers having a fiber diameter of 1 μm or less havebeen known, as well as cellulose fibers having a fiber diameter of 10 μmor more and 50 μm or less.

Cellulose fibers are obtained by subjecting a wood or non-wood materialto a digestion treatment. There is a case where a bleaching step isestablished after the digestion treatment step, and by performing such ableaching step, cellulose fibers having high whiteness are obtained.Moreover, by establishing such a bleaching step, deinked pulp(reproduced cellulose fibers) can be obtained from a printed wastepaper.

Patent Document 1 discloses a method for producing a deinked pulp,comprising a step of bleaching a printed waste paper. In this method,hydrogen peroxide, hydrosulfite, thiourea dioxide, sodium thiosulfateand the like are used as bleaching drugs for reproducing the printedwaste paper. Studies are conducted to obtain a deinked pulp having highwhiteness by adjusting bleaching conditions, as appropriate. PatentDocument 2 discloses a method for producing a cellulose pulp, comprisinga bleaching step in which chlorine dioxide and a bleaching aid are used.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2016-125153

Patent Document 2: JP-A-2009-108430

SUMMARY OF INVENTION Object to be Solved by the Invention

By the way, studies have been conducted to control the physicalproperties of cellulose fibers by introducing ionic substituents such asphosphoric acid groups into the cellulose fibers. Moreover, studies havealso been conducted to obtain ultrafine cellulose fibers by usingcellulose fibers having ionic substituents such as phosphoric acidgroups. However, there has been a case where cellulose fibers havingphosphoric acid groups have insufficient whiteness, and therefore, ithas been desired to improve the whiteness of the cellulose fibers havingphosphoric acid groups.

In order to solve the above-described problem, the present inventionaims to optimize a bleaching process in a step of producing cellulosefibers having phosphoric acid groups, so as to provide a pulp comprisingcellulose fibers having high whiteness. Further, it is another object ofthe present invention to provide a slurry capable of realizing a sheetwith low yellowness, a sheet with low yellowness, and a laminate havingthe same.

Means for Solving the Object

The present inventors have found that, by optimizing a bleaching processin a step of producing phosphorylated cellulose fibers, the whiteness ofa pulp comprising cellulose fibers can be enhanced, even though theamount of phosphoric acid groups introduced is large in the pulpcomprising cellulose fibers.

Specifically, the present invention has the following configurations.

[1] A pulp comprising cellulose fibers having 0.5 mmol/g or more ofphosphoric acid groups or phosphoric acid group-derived substituents,wherein when the pulp is processed into a sheet under the followingcondition A and four sheets are then laminated on one another, the ISOwhiteness of the obtained laminate measured in accordance with JIS P8148, with the exception that the test piece defined by JIS P 8148 isset to be the obtained laminate, is 82% or more:

(Condition A)

ion exchange water is added to the pulp to prepare a suspension in whichthe concentration of the pulp comprising phosphorylated cellulose fibersis 0.3% by mass, and then, a wet sheet having an absolute dry basisweight of 200 g/m² is formed from the suspension; the wet sheet ispeeled off from a filter, is then placed on a stainless steel tray, andis then dried under conditions of 23° C. and a relative humidity of 50%for 3 days; and both surfaces of the dried sheet are sandwiched bypapers and metal plates, and the sheet is then pressed by a pressure of7.7 MPa for 1 minute to obtain a pulp sheet.

[2] The pulp according to [1], wherein when the pulp is processed into asheet under the condition A and four sheets are then laminated on oneanother, the b* value of the obtained laminate according to the L*a*b*color system is 5.5 or less.

[3] The pulp according to [1] or [2], wherein when a fibrillationtreatment and a centrifugation treatment are carried out under thefollowing condition a, a supernatant yield calculated according to thefollowing Equation b is 50% or more:

(Condition a)

-   -   the pulp is diluted with ion exchange water to a concentration        of 0.5% by mass to obtain a slurry, and the slurry is then        subjected to a fibrillation treatment using CLEARMIX-2.2S        manufactured by M Technique Co., Ltd., at a rotation number of        21500 rpm for 30 minutes; and thereafter, the obtained slurry is        diluted with ion exchange water to a solid concentration of 0.1%        by mass, and the resulting slurry is then subjected to a        centrifugation treatment at 12000 G for 10 minutes,

Supernatant yield (%)=solid concentration (% by mass) in supernatantobtained after centrifugation treatment/solid concentration (% by mass)in slurry before centrifugation treatment×100.   (b)

[4] A slurry comprising phosphorylated ultrafine cellulose fibers, whichhave 0.5 mmol/g or more of phosphoric acid groups or phosphoric acidgroup-derived substituents and have a fiber width of 1000 nm or less,wherein

-   -   when a sheet is formed using the slurry under the following        condition c, the yellowness (YIN) of the sheet relative to a        film thickness of 30 um calculated according to the following        Equation d is 0.57 or less:

(Condition c)

-   -   the concentration of the phosphorylated ultrafine cellulose        fibers in the slurry is adjusted to be 0.5% by mass, and the        slurry is then dehydrated by suction filtration using, as a        filter medium, a PVDF membrane filter having a pore diameter of        650 nm, until the solid content in the phosphorylated ultrafine        cellulose fibers becomes 4% by mass or more; and thereafter, the        resulting slurry is subjected to drying under tension for 2 days        in a humidity conditioning chamber at 23° C. and a relative        humidity of 50%, so as to obtain a sheet having an absolute dry        basis weight of 40 g/m²,

Yellowness (YI₃₀) of sheet relative to film thickness of 30μm=yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm)),  (d)

wherein, in the above equation, the yellowness (YI) of a sheet ismeasured in accordance with JIS K 7373.

[5] A sheet comprising phosphorylated ultrafine cellulose fibers, whichhave 0.5 mmol/g or more of phosphoric acid groups or phosphoric acidgroup-derived substituents and have a fiber width of 1000 nm or less,wherein

-   -   YI₃₀ calculated according to the following Equation d is 0.57 or        less:

Yellowness (YI₃₀) of sheet relative to film thickness of 30μm=yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm)),  (d)

wherein, in the above equation, the yellowness (YI) of a sheet ismeasured in accordance with JIS K 7373

[6] A laminate having the sheet according to [5] and a resin layerdisposed on at least one surface of the sheet.

[7] A method for producing a pulp, comprising bleaching a pulpcomprising cellulose fibers having 0.5 mmol/g or more of phosphoric acidgroups or phosphoric acid group-derived substituents.

[8] The method for producing a pulp according to [7], wherein adifferent between the amount of phosphoric acid groups in cellulosefibers before the bleaching and the amount of phosphoric acid groups incellulose fibers after the bleaching is 0.2 mmol/g or less.

[9] The method for producing a pulp according to [7] or [8], wherein adifference between the viscosity average polymerization degree ofultrafine cellulose fibers obtained by subjecting a pulp comprisingcellulose fibers before the bleaching to a fibrillation treatmentperformed under the following condition e, and the viscosity averagepolymerization degree of ultrafine cellulose fibers obtained bysubjecting a pulp comprising cellulose fibers after the bleaching to afibrillation treatment performed under the following condition e, is 100or less:

(Condition e)

-   -   a pulp comprising cellulose fibers is diluted with ion exchange        water to a concentration of 0.5% by mass, so as to obtain a        slurry, and the slurry is then subjected to a fibrillation        treatment using CLEARMIX-2.2S manufactured by M Technique Co.,        Ltd., at a rotation number of 21500 rpm for 30 minutes.

[10] The method for producing a pulp according to any one of [7] to [9],wherein

-   -   when a pulp comprising cellulose fibers before the bleaching is        subjected to a fibrillation treatment performed under the        following condition e so as to obtain an ultrafine cellulose        fiber-containing slurry 1 having a concentration of 0.4% by        mass, the viscosity of the slurry 1 is defined as P, and also,    -   when a pulp comprising cellulose fibers after the bleaching is        subjected to a fibrillation treatment performed under the        following condition e so as to obtain an ultrafine cellulose        fiber-containing slurry 2 having a concentration of 0.4% by        mass, the viscosity of the slurry 2 is defined as Q, and    -   the P/Q value is 0.5 or more and 2.0 or less:

(Condition e)

-   -   the pulp is diluted with ion exchange water to a concentration        of 0.5% by mass, so as to obtain a slurry, and the slurry is        then subjected to a fibrillation treatment using CLEARMIX-2.2S        manufactured by M Technique Co., Ltd., at a rotation number of        21500 rpm for 30 minutes.

Advantageous Effects of Invention

In the present invention, a bleaching process has been successfullyoptimized in a step of producing phosphorylated cellulose fibers.According to the present invention, a pulp comprising cellulose fibers,in which the amount of phosphoric acid groups introduced is large andwhich have sufficiently high whiteness, can be obtained. According tothe present invention, a slurry capable of realizing a sheet with lowyellowness, a sheet with low yellowness, and a laminate having the sameare further provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a graph showing the relationship between the amount of NaOHadded dropwise to a fiber raw material and the pH.

EMBODIMENTS OF CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail. Thebelow-mentioned constituent features will be explained based onrepresentative embodiments or specific examples in some cases. However,the present invention is not limited to such embodiments.

(Pulp)

The present invention relates to a pulp comprising cellulose fibershaving 0.5 mmol/g or more of phosphoric acid groups or phosphoric acidgroup-derived substituents, wherein when the pulp is processed into asheet under the following condition A and four sheets are then laminatedon one another, the ISO whiteness of the obtained laminate measured inaccordance with JIS P 8148, with the exception that the test piecedefined by JIS P 8148 is set to be the obtained laminate, is 82% ormore.

(Condition A)

Ion exchange water is added to the pulp to prepare a suspension in whichthe concentration of the pulp comprising phosphorylated cellulose fibersis 0.3% by mass, and then, a wet sheet having an absolute dry basisweight of 200 g/m² is formed from the suspension. The wet sheet ispeeled off from a filter, is then placed on a stainless steel tray, andis then dried under conditions of 23° C. and a relative humidity of 50%for 3 days. Both surfaces of the dried sheet are sandwiched by papersand metal plates, and the sheet is then pressed by a pressure of 7.7 MPafor 1 minute to obtain a pulp sheet.

In the present invention, a pulp comprising cellulose fibers, into whichlarge quantities of phosphoric acid groups are introduced, and which hassufficiently high whiteness, can be obtained. Moreover, in the presentinvention, such a pulp having high whiteness is fibrillated, so thatultrafine cellulose fibers capable of exhibiting desired physicalproperties can be obtained.

In a phosphorylation step of introducing phosphoric acid groups intocellulose fibers, color developed in the cellulose fibers should beimproved. In particular, when many phosphoric acid groups are introducedinto the cellulose fibers, such coloration tends to be increased, andthus, the improvement thereof is desired. Herein, in order to reducecoloration in the phosphorylated cellulose fibers, bleaching of a pulpcomprising phosphorylated cellulose fibers is considered. However, uponbleaching the pulp comprising cellulose fibers, there is a case wherecellulose fibers themselves are hydrolyzed, or the physical propertiesthereof are changed. That is to say, by establishing a bleaching step,cellulose fibers may be damaged, and as a result, there is a concernthat the physical properties of the damaged cellulose fibers or a pulpcomprising the damaged cellulose fibers would be greatly changed. Forexample, when a pulp comprising phosphorylated cellulose fibers isfibrillated, a resulting ultrafine cellulose fiber-containing slurryexhibits high transparency and high viscosity. However, in the case ofusing a pulp comprising phosphorylated cellulose fibers obtained after ableaching step, there is a concern that fibrillation does not favorablyprogress because the phosphorylated cellulose fibers are damaged, andthus that the transparency and viscosity of the obtained ultrafinecellulose fiber-containing slurry are decreased. That is, it has beenconsidered that the fibrillation of a pulp comprising phosphorylatedcellulose fibers would be prevented by giving damage to the cellulosefibers.

In general, a pulp comprising phosphorylated cellulose fibers isobtained by phosphorylating a raw material pulp before introduction ofphosphoric acid groups. Such a raw material pulp is obtained bydigesting chipped wood materials such as hard wood or soft wood, orchipped non-wood materials such as herbaceous plants, with a digestionliquid consisting of caustic soda or sodium sulfide, and then performinga bleaching treatment on the resulting materials. As such a raw materialpulp, a pulp, from which coloration-causing substances such as ligninhave been removed, is generally used. Thus, the obtained raw materialpulp is mixed with a phosphorylating agent, and the mixture is thensubjected to a heat treatment, etc., to obtain a pulp comprisingphosphorylated cellulose fibers. In the phosphorylation step, for thepurpose of efficiently performing phosphorylation, a heat treatment iscarried out. However, as a result of the studies conducted by thepresent inventors, it has been found that the obtained pulp comprisingphosphorylated cellulose fibers is turned yellow due to the influence ofthe heat treatment and the phosphorylating agent. In addition, it isalso concerned that lignin remaining in the digestion step would causecoloration in the subsequent phosphorylation step.

When the amount of phosphoric acid groups introduced into cellulosefibers intends to be increased, the phosphorylation step may be carriedout multiple times, or a heating time may be prolonged. Hence, a pulpcomprising cellulose fibers, into which large quantities of phosphoricacid groups are introduced, is problematic in that whiteness is easilydecreased (deteriorated).

Moreover, in steps performed after large quantities of phosphoric acidgroups have been introduced into cellulose fibers, chemical treatmentsother than a pH adjustment step are not generally carried out. This isbecause the amount of the introduced phosphoric acid groups and thepolymerization degree of cellulose fibers are retained. As such, if ableaching treatment has been performed once upon obtaining a rawmaterial pulp, in general, a bleaching treatment step is not establishedagain after completion of the phosphorylation treatment step. That is tosay, when a pulp comprising phosphorylated cellulose fibers having highwhiteness intends to be produced, no dominant means have been foundother than searching conditions for reducing coloration to the minimumin the phosphorylation step.

In the present invention, a bleaching treatment is purposely performedafter completion of the phosphorylation treatment step. In the presentinvention, it has been found that, even in a case where a bleaching stepis established after the phosphorylation treatment step, the amount ofthe introduced phosphoric acid groups can be maintained at a high level.Moreover, in the present invention, by establishing a bleaching stepafter the phosphorylation treatment step, a pulp comprisingphosphorylated cellulose fibers having high whiteness can be obtained.In the present invention, it has been found that the physical propertiesor characteristics of the thus obtained pulp comprising phosphorylatedcellulose fibers having high whiteness are not damaged by the bleachingstep. Specifically, even in a case where a bleaching step is establishedafter the phosphorylation treatment step, a pulp comprisingphosphorylated cellulose fibers can be favorably fibrillated, and aslurry containing ultrafine cellulose fibers obtained by fibrillationcan exhibit desired physical properties. For example, a slurrycontaining ultrafine cellulose fibers has high transparency and canexhibit high viscosity.

In the present invention, by performing a bleaching treatment using asuitable amount of bleaching agent under suitable bleaching conditions,a reduction in the amount of phosphoric acid groups contained inphosphorylated cellulose fibers and the polymerization degree thereofcan be suppressed. In the present invention, while the object of thepresent invention that is the improvement of whiteness is achieved, thetype and amount of a bleaching agent and the temperature applied duringthe bleaching treatment, which do not cause a reduction in the amount ofphosphoric acid groups contained in phosphorylated cellulose fibers andthe polymerization degree thereof, have been found based on theaforementioned findings.

As mentioned above, in the present invention, a method for producing apulp comprising cellulose fibers having high whiteness, into which largequantities of phosphoric acid groups are introduced, has been found.Moreover, the pulp comprising phosphorylated cellulose fibers havinghigh whiteness of the present invention is not prevented from beingfibrillated. Thus, the pulp comprising phosphorylated cellulose fibershaving high whiteness of the present invention provides a highlytransparent slurry after it has been fibrillated, and the slurryexhibits high viscosity, as with a pulp comprising cellulose fibers,which are not subjected to a bleaching step after completion of thephosphorylation step.

The content of the phosphoric acid groups or phosphoric acidgroup-derived substituents (hereinafter simply referred to as“phosphoric acid groups” at times) comprised in the cellulose fibers maybe 0.5 mmol/g or more per gram (mass) of the cellulose fibers, and it ismore preferably 1.00 mmol/g or more, and further preferably 1.10 mmol/gor more, per gram (mass) of the cellulose fibers. On the other hand, thecontent of the phosphoric acid groups is preferably 3.65 mmol/g or less,more preferably 3.5 mmol/g or less, and further preferably 3.0 mmol/g orless. Besides, in the present description, the content of the phosphoricacid groups in the cellulose fibers is equal to the amount of stronglyacidic groups of phosphoric acid groups in the cellulose fibers, asdescribed later. In the present invention, it is preferable that thecontent of the phosphoric acid groups in a pulp comprising cellulosefibers be also within the above-described range.

The content of the phosphoric acid groups in the cellulose fibers can bemeasured by a neutralization titration method. Upon the measurement bysuch a neutralization titration method, phosphoric acid groups arecompletely converted to acid type groups, and fibrillation is thenperformed by a mechanical treatment step (fibrillation step).Thereafter, while a sodium hydroxide aqueous solution is added to theobtained ultrafine cellulose fiber-containing slurry, changes in the pHare obtained, so that the amount of phosphoric acid groups introducedcan be measured.

Conversion of the phosphoric acid groups to acid type groups is carriedout by diluting the obtained phosphorylated cellulose fibers with ionexchange water, so that the concentration of cellulose fibers becomes 2%by mass, and then gradually adding a sufficient amount of 1 Nhydrochloric acid aqueous solution to the resulting phosphorylatedcellulose fibers, while stirring. In such conversion of the phosphoricacid groups to acid type groups, it is preferable to repeat theoperation of dehydrating the above-described cellulose fiber-containingslurry to obtain a dehydrated sheet, then diluting the dehydrated sheetwith ion exchange water again, and then adding a 1 N hydrochloric acidaqueous solution to the resultant, so that the phosphoric acid groupscontained in the cellulose fibers can be completely converted to acidtype groups. Then, after completion of the step of converting thephosphoric acid groups to acid type groups, it is preferable to repeatthe operation of stirring the obtained cellulose fiber-containing slurryto uniformly disperse it, followed by filtration and dehydration toobtain a dehydrated sheet, so that redundant hydrochloric acid can befully washed away.

In the mechanical treatment step (fibrillation step), ion exchange wateris poured onto the obtained dehydrated sheet to obtain a cellulosefiber-containing slurry, in which the concentration of cellulose fibersis 0.3% by mass, and this slurry is then treated using a defibrationtreatment device (manufactured by M Technique Co., Ltd., CLEARMIX-2.2S)under conditions of 21500 rpm for 30 minutes. Thus, an ultrafinecellulose fiber-containing slurry is obtained.

In the titration using alkali, changes in the pH values indicated by thedispersion are measured while adding a 0.1 N sodium hydroxide aqueoussolution to the ultrafine cellulose fiber-containing slurry. In thisneutralization titration, in a curve obtained by plotting pH valuesmeasured with respect to the amount of alkali (sodium hydroxide aqueoussolution) added, two points, in which the increment (the derivative ofpH to the amount of alkali added dropwise) becomes maximum, are obtained(i.e., a point in which the increment becomes maximum, and a point inwhich the increment becomes second maximum). Among these, the amount ofalkali required until the maximum point of the increment obtained firstafter addition of alkali (hereinafter referred to as a “first endpoint”) is equal to the amount of strongly acidic groups in thedispersion used in the titration, and the amount of alkali requireduntil the maximum point of the increment obtained second after additionof alkali (hereinafter referred to as a “second end point”) is equal tothe amount of weakly acidic groups in the dispersion used in thetitration. The alkali amount (mmol) required until the first end pointis divided by the solid content (g) in the ultrafine cellulosefiber-containing slurry to be titrated, to obtain a first dissociatedalkali amount (mmol/g). This amount is defined to be the content of thephosphoric acid groups in the cellulose fibers.

FIG. 1 shows a curve obtained by plotting the pH values measured withrespect to the amount of alkali (sodium hydroxide aqueous solution) inneutralization titration. The region up to the first end point isreferred to as a first region, and the region up to the second end pointis referred to as a second region. Besides, after the second region,there is a third region. In short, three regions appear. In FIG. 1, theamount of the alkali required for the first region is equal to theamount of strongly acidic groups in the slurry used in the titration,and the amount of the alkali required for the second region is equal tothe amount of weakly acidic groups in the slurry used in the titration.

The pulp comprising cellulose fibers of the present invention isprocessed into a sheet under the aforementioned condition A and foursheets are then laminated on one another, the ISO whiteness of theobtained laminate measured in accordance with JIS P 8148, with theexception that the test piece defined by JIS P 8148 is set to be theobtained laminate, is 82% or more, preferably 83% or more, and morepreferably 84% or more. In addition, the ISO whiteness of the sheets mayalso be 100%. As an apparatus for measuring ISO whiteness, a whitenessspectrophotometer (manufactured by Suga Test Instruments Co., Ltd.,SC-10WN) can be used.

In the present invention, four sheets formed under the followingcondition A are laminated on one another, and the ISO whiteness thereofis then measured.

(Condition A)

Ion exchange water is added to the pulp to prepare a suspension in whichthe concentration of the pulp comprising phosphorylated cellulose fibersis 0.3% by mass, and then, a wet sheet having an absolute dry basisweight of 200 g/m² is formed from the suspension. The wet sheet ispeeled off from a filter, is then placed on a stainless steel tray, andis then dried under conditions of 23° C. and a relative humidity of 50%for 3 days. Both surfaces of the dried sheet are sandwiched by papersand metal plates, and the sheet is then pressed by a pressure of 7.7 MPafor 1 minute to obtain a pulp sheet. Besides, the dried sheet can bepressed, for example, using a mini-hot press (manufactured by Toyo SeikiKogyo Co., Ltd., MP-SNH).

The pulp comprising cellulose fibers of the present invention isprocessed into a sheet under the above-described condition A and foursheets are then laminated on one another. The b* value of the obtainedlaminate according to the L*a*b* color system is preferably 5.5 or less,more preferably 5.0 or less, further preferably 4.6 or less, andparticularly preferably 4.0 or less. The b* value can be measured by thesame method as that for measuring the aforementioned ISO whiteness. Whenthe pulp comprising cellulose fibers of the present invention isprocessed into a sheet and four sheets are then laminated on oneanother, the b* value of the laminate is within the above-describedrange, and therefore, exhibition of yellow color is suppressed. Thereby,the whiteness of the sheet is enhanced.

In the pulp comprising cellulose fibers of the present invention, evenin a case where a bleaching step is established after completion of thephosphorylation step, damage on the cellulose fibers is suppressed.Hence, the cellulose fibers can be fibrillated, as with cellulose fibersthat are not subjected to a bleaching step after completion of thephosphorylation step. That is to say, when the pulp comprising cellulosefibers of the present invention is fibrillated, the yield of ultrafinecellulose fibers is high. Specifically, when a fibrillation treatmentand a centrifugation treatment are carried out under the followingcondition a, the supernatant yield calculated according to the followingEquation b is preferably 50% or more. The supernatant yield is morepreferably 70% or more, even more preferably 80% or more, furtherpreferably 90% or more, and particularly preferably 95% or more.

(Condition a)

The pulp is diluted with ion exchange water to a concentration of 0.5%by mass to obtain a slurry, and the slurry is then subjected to afibrillation treatment using CLEARMIX-2.2S manufactured by M TechniqueCo., Ltd., at a rotation number of 21500 rpm for 30 minutes. Thereafter,the obtained slurry is diluted with ion exchange water to a solidconcentration of 0.1% by mass, and the resulting slurry is thensubjected to a centrifugation treatment at 12000 G for 10 minutes.

Supernatant yield (%)=solid concentration (% by mass) in supernatantobtained after centrifugation treatment/solid concentration (% by mass)in slurry before centrifugation treatment×100.   (b)

Besides, in the present invention, the fiber width of a cellulose fiberis not particularly limited. The fiber width of a cellulose fiber may begreater than 1000 nm, or may also be 1000 nm or less. Moreover,cellulose fibers having a fiber width of greater than 1000 nm may bepresent together with cellulose fibers having a fiber width of 1000 nmor less. In the present description, when the fiber width of a cellulosefiber is 1000 nm or less, such a cellulose fiber is referred to as an“ultrafine cellulose fiber.”

Herein, the fiber width of a cellulose fiber can be measured by electronmicroscopic observation according to the following method. First, anaqueous suspension of cellulose fibers having a concentration of 0.05%by mass or more and 0.1% by mass or less is prepared, and the suspensionis casted onto a hydrophilized carbon film-coated grid as a sample forTEM observation. At this time, SEM images of the surface of thesuspension casted onto glass may be observed. The sample is observedusing electron microscope images taken at a magnification of 1000×,5000×, 10000×, or 50000×, depending on the widths of the constituentfibers. However, the sample, the observation conditions, and themagnification are adjusted so as to satisfy the following conditions:

(1) A single straight line X is drawn in any given portion in anobservation image, and 20 or more fibers intersect with the straightline X.

(2) A straight line Y, which intersects perpendicularly with theaforementioned straight line in the same image as described above, isdrawn, and 20 or more fibers intersect with the straight line Y

The widths of the fibers intersecting the straight line X and thestraight line Y in the observation image meeting the above-describedconditions are visually read. Three or more sets of images of surfaceportions, which are at least not overlapped, are thus observed, and thewidths of the fibers intersecting the straight line X and the straightline Y are read in the each image. At least 120 fiber widths (20fibers×2×3=120) are thus read.

The average fiber length of cellulose fibers is not particularlylimited, but it is preferably 0.1 μm or more, more preferably 1 μm ormore, even more preferably 0.1 mm or more, and further preferably 0.6 mmor more. On the other hand, it is preferably 5 mm or less, and morepreferably 2 mm or less. Herein, the average fiber length of cellulosefibers can be obtained, for example, by using Kajaani Fiber SizeAnalyzer (FS-200) manufactured by Kajaani Automation to measure thelength weighted average fiber length. Otherwise, the average fiberlength of cellulose fibers may also be measured by using a scanningelectron microscope (SEM), a transmission electron microscope (TEM),etc., depending on the length of the fiber.

When the cellulose fibers are ultrafine cellulose fibers, the ultrafinecellulose fibers preferably have a type I crystal structure. In thisregard, the fact that ultrafine cellulose fibers have a type I crystalstructure may be identified by a diffraction profile obtained from awide angle X-ray diffraction photograph using CuKα (λ=1.5418 Å)monochromatized with graphite. Specifically, it may be identified basedon the fact that there are typical peaks at two positions near 2θ=14° ormore and 17° or less, and near 2θ=22° or more and 23° or less.

The percentage of the type I crystal structure occupied in the ultrafinecellulose fibers is preferably 30% or more, more preferably 50% or more,and further preferably 70% or more. In this case, more excellentperformance can be expected, in terms of heat resistance and theexpression of low linear thermal expansion. The crystallinity can beobtained by measuring an X-ray diffraction profile and obtaining itaccording to a common method (Seagal et al., Textile Research Journal,Vol. 29, p. 786, 1959).

In the present description, cellulose fibers have phosphoric acid groups(phosphoric acid groups or phosphoric acid group-derived substituents).In the present invention, such cellulose fibers may also be referred toas “phosphorylated cellulose fibers,” “phosphorylated cellulose,” or“phosphorylated cellulose fibers.”

The phosphoric acid group comprised in the phosphorylated cellulosefibers is a divalent functional group corresponding to a phosphoric acidfrom which a hydroxyl group is removed. Specifically, it is a grouprepresented by —PO₃H₂. The phosphoric acid group-derived substituentsinclude substituents, such as condensation-polymerized phosphoric acidgroups, salts of phosphoric acid groups, and phosphoric acid estergroups, and they may preferably be ionic substituents.

In the present invention, the phosphoric acid group or the phosphoricacid group-derived substituent may be a substituent represented by thefollowing Formula (1):

In the Formula (1), a, b, and n each represent a natural number(provided that a=b×m); at least one of α1, α2, . . . , αn and α′ is O⁻,and the rest are either R or OR. All of αn and α′ may also be O⁻. When nis 2 or greater and α′ is R or OR, at least one of αn is O⁻ and the restare R or OR. When n is 2 or greater and α′ is O⁻, all of αn may be R orOR, or at least one of an may be O⁻ and the rest may be R or OR. R eachrepresents a hydrogen atom, a saturated straight chain hydrocarbongroup, a saturated branched chain hydrocarbon group, a saturated cyclichydrocarbon group, an unsaturated straight chain hydrocarbon group, anunsaturated branched chain hydrocarbon group, an unsaturated cyclichydrocarbon group, an aromatic group, or a derivative group thereof

Examples of the saturated straight chain hydrocarbon group include amethyl group, an ethyl group, an n-propyl group, and an n-butyl group,but are not particularly limited thereto. Examples of the saturatedbranched chain hydrocarbon group include an i-propyl group and a t-butylgroup, but are not particularly limited thereto. Examples of thesaturated cyclic hydrocarbon group include a cyclopentyl group and acyclohexyl group, but are not particularly limited thereto. Examples ofthe unsaturated straight chain hydrocarbon group include a vinyl groupand an allyl group, but are not particularly limited thereto. Examplesof the unsaturated branched chain hydrocarbon group include ani-propenyl group and a 3-butenyl group, but are not particularly limitedthereto. Examples of the unsaturated cyclic hydrocarbon group include acyclopentenyl group and a cyclohexenyl group, but are not particularlylimited thereto. Examples of the aromatic group include a phenyl groupand a naphthyl group, but are not particularly limited thereto.

Moreover, examples of the derivative of the above-described R includefunctional groups such as a carboxyl group, a hydroxyl group or an aminogroup, in which at least one type selected from functional groups isadded to or substituted with the main chain or side chain of theabove-described various types of hydrocarbon groups, but are notparticularly limited thereto. Furthermore, the number of carbon atomsconstituting the main chain of the above-described R is not particularlylimited, but it is preferably 20 or less, and more preferably 10 orless. If the number of carbon atoms constituting the main chain of the Rexceeds 20, the molecules of phosphorus oxoacid groups containing Rbecome too large, the groups can hardly permeate into a fiber rawmaterial, so that the yield of ultrafine cellulose fibers is likely tobe decreased.

βb+ is a mono- or more-valent cation consisting of an organic orinorganic matter. Examples of the mono- or more-valent cation consistingof an organic matter include an aliphatic ammonium and an aromaticammonium, and examples of the mono- or more-valent cation consisting ofan inorganic matter include alkali metal ions such as sodium, potassiumor lithium ions, divalent metal cations such as calcium or magnesiumions, and hydrogen ions, but are not particularly limited thereto. Thesecan be applied alone as a single type or in combination of two or moretypes. As such mono- or more-valent cations consisting of an organic orinorganic matter, sodium or potassium ions, which hardly cause theyellowing of a fiber raw material containing β upon heating and areindustrially easily applicable, are preferable, but are not particularlylimited thereto.

(Slurry)

The present invention relates to a slurry produced using theaforementioned pulp comprising cellulose fibers. In the presentdescription, such a slurry is also referred to as a “cellulosefiber-containing slurry.” The cellulose fibers contained in the slurryof the present invention are preferably ultrafine cellulose fibers. Thatis to say, the slurry of the present invention preferably relates to aslurry containing phosphorylated ultrafine cellulose fibers having 0.5mmol/g or more of phosphoric acid groups or phosphoric acidgroup-derived substituents and also having a fiber width of 1000 nm orless.

When a sheet is formed using the slurry of the present invention underthe following condition c, the yellowness (YI₃₀) of the sheet relativeto a film thickness of 30 μm, which is calculated according to thefollowing Equation d, is 0.57 or less. The yellowness (YI₃₀) of thesheet relative to a film thickness of 30 μm is preferably 0.55 or less,more preferably 0.50 or less, and further preferably 0.45 or less.

(Condition c)

-   -   the concentration of the phosphorylated ultrafine cellulose        fibers in the slurry is adjusted to be 0.5% by mass, and the        slurry is then dehydrated by suction filtration using, as a        filter medium, a PVDF membrane filter having a pore diameter of        650 nm, until the solid content in the phosphorylated ultrafine        cellulose fibers becomes 4% by mass or more. Thereafter, the        resulting slurry is subjected to drying under tension for 2 days        in a humidity conditioning chamber at 23° C. and a relative        humidity of 50%, so as to obtain a sheet having an absolute dry        basis weight of 40 g/m².

Yellowness (YI₃₀) of sheet relative to film thickness of 30μm=yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm))  (d)

In the above equation, the yellowness (YI) of a sheet is measured inaccordance with JIS K 7373. The yellowness (YI) of the sheet is themeasured value of the yellowness (YI) of the sheet formed under theabove condition c.

To date, there has been a case where a sheet produced using a slurrycontaining ultrafine cellulose fibers is required to suppress theyellowness thereof. According to the present invention, a slurry capableof realizing a sheet with low yellowness can be obtained, as mentionedabove. This is considered because of the use of a pulp with highwhiteness, in which exhibition of yellow color is suppressed.

The viscosity of a phosphorylated ultrafine cellulose fiber-containingslurry (hereinafter also referred to as an “ultrafine cellulosefiber-containing slurry”) at 25° C. is preferably 8000 mPa·s or more,more preferably 10000 mPa·s or more, and further preferably 11000 mPa·sor more. The upper limit of the viscosity of the ultrafine cellulosefiber-containing slurry at 25° C. is not particularly limited, but itcan be set at, for example, 50000 mPa·s.

Herein, the viscosity of the ultrafine cellulose fiber-containing slurryis measured by the following procedures. First, the ultrafine cellulosefiber-containing slurry is diluted to result in a solid concentration of0.4% by mass, and the resulting slurry is then homogeneously stirredusing a disperser at 1500 rpm. The obtained slurry is left at rest for24 hours, and thereafter, the viscosity of the slurry is measured usinga type B viscometer (manufactured by BROOKFIELD; analog viscometerT-LVT). Regarding measurement conditions, the slurry is rotated at 25°C. at 3 rpm for 3 minutes, and the viscosity thereof is then measured.

The haze of the ultrafine cellulose fiber-containing slurry ispreferably 10% or less, more preferably 5% or less, and furtherpreferably 3% or less. The haze of the ultrafine cellulosefiber-containing slurry may also be 0%.

Herein, the haze of the ultrafine cellulose fiber-containing slurry ismeasured by the following procedures. First, the ultrafine cellulosefiber-containing slurry is diluted with ion exchange water to result ina solid concentration of 0.2% by mass, and then, the resulting slurry ishomogeneously stirred. Thereafter, the slurry is placed in a glass cellfor liquid having an optical path length of 1 cm (manufactured byFujiwara Scientific Co., Ltd.; MG-40; inverse optical path), and thehaze of the slurry is then measured in accordance with JIS K 7136, usinga hazemeter (manufactured by MURAKAMI COLOR RESEARCH LABORATORY Co.,Ltd.; HM-150). The zero point is measured with ion exchange water whichwas placed in the glass cell.

The polymerization degree of ultrafine cellulose fibers contained in theultrafine cellulose fiber-containing slurry is preferably 500 or more,more preferably 600 or more, further preferably 700 or more, andparticularly preferably 800 or more. On the other hand, thepolymerization degree of the ultrafine cellulose fibers is preferably2000 or less. Since the above-described polymerization degree is anaverage polymerization degree measured according to a viscosity methodas described later, it is also referred to as a “viscosity averagepolymerization degree.”

The polymerization degree of the ultrafine cellulose fibers iscalculated from the viscosity of a pulp measured in accordance withTappi T230. Specifically, the ultrafine cellulose fibers as ameasurement target are dispersed in a dispersion medium, the viscositythereof is then measured (defined as η1), and the blank viscosity isthen measured using only the dispersion medium (defined as η0).Thereafter, a specific viscosity (ηsp) and an intrinsic viscosity ([η])are calculated according to the following equations.

ηsp=(η1/η0)−1

[η]=ηsp/(c(1+0.28×ηsp))

In the above equation, c indicates the concentration of ultrafinecellulose fibers upon the measurement of the viscosity.

Further, polymerization degree (DP) is calculated according to thefollowing equation.

DP=1.75×[η]

The cellulose fiber-containing slurry may further comprise otheroptional components. Examples of such optional components may includeantifoaming agents, lubricants, surfactants, ultraviolet absorbingagents, dyes, pigments, fillers, stabilizers, organic solvents misciblewith water, such as alcohol, antiseptics, organic fine particles,inorganic fine particles, and resins (pellet-type and fibrous resins).Besides in the present description, a pulp comprising cellulose fibersmay comprise hemicellulose, lignin, a resin component (an extractedcomponent such as terpene) and ash, as well as cellulose fibers.

(Method for Producing a Pulp Comprising Cellulose Fibers)

The present invention relates to a method for producing a pulp,comprising a step of bleaching a pulp comprising cellulose fibers having0.5 mmol/g or more of phosphoric acid groups or phosphoric acidgroup-derived substituents. Provided that the obtained pulp comprisingcellulose fibers is processed into a sheet under the above-describedcondition A and then, the four sheets obtained under the above-describedcondition A are laminated on one another to form a test piece defined byJIS P 8148, the method for producing a pulp of the present invention isidentical to the method for producing a pulp, in which the obtainedlaminate has an ISO whiteness of 82% or more, wherein the ISO whitenessis measured in accordance with JIS P 8148.

<Raw Material Pulp>

The raw material pulp for obtaining a pulp comprising cellulose fibersis not particularly limited, but the raw material pulp can be a woodpulp, a non-wood pulp, and a deinked pulp. Examples of the wood pulpinclude chemical pulps such as leaf bleached kraft pulp (LBKP), needlebleached kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP),soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraftpulp (OKP). Further, included are, but not particularly limited to,semichemical pulps such as semi-chemical pulp (SCP) and chemi-groundwood pulp (CGP); and mechanical pulps such as ground pulp (GP) andthermomechanical pulp (TMP, BCTMP). Examples of the non-wood pulpinclude, but not particularly limited to, cotton pulps such as cottonlinter and cotton lint; non-wood type pulps such as hemp, wheat straw,and bagasse; and cellulose isolated from ascidian, seaweed, etc.,chitin, and chitosan. As a deinked pulp, there is a deinked pulp using awaste paper as a raw material, but it is not particularly limitedthereto. The pulp of the present embodiment may be used singly, or incombination of two or more types. Among the above-listed pulp types, awood pulp and a deinked pulp including cellulose are preferable from theviewpoint of easy availability.

The raw material pulp is obtained by digesting chipped wood materialssuch as hard wood or soft wood, or chipped non-wood materials such asherbaceous plants, with a digestion liquid consisting of caustic soda orsodium sulfide, and then performing a bleaching treatment on theresulting materials. As such a raw material pulp, a pulp, from whichcoloration-causing substances such as lignin have been removed, is used.

<Phosphoric Acid Group Introduction Step>

The phosphoric acid group introduction step is a step of introducingphosphoric acid groups into cellulose fibers comprised in the rawmaterial pulp obtained as described above. The phosphoric acid groupintroduction step may be performed by allowing at least one selectedfrom a compound having phosphoric acid groups or phosphoric acidgroup-derived substituents and salts thereof (hereinafter, referred toas a “phosphorylating agent” or “Compound A”) to react with a pulpcomprising cellulose fibers. Such a phosphorylating agent may be mixedinto the pulp comprising cellulose fibers in a dry or wet state, in theform of a powder or an aqueous solution. In another example, a powder oran aqueous solution of the phosphorylating agent may be added into aslurry of the pulp comprising cellulose fibers. That is to say, thephosphoric acid group introduction step includes, at least, a step ofmixing a pulp comprising cellulose fibers with a phosphorylating agent.

The phosphoric acid group introduction step may be performed by allowinga phosphorylating agent to react with a pulp comprising cellulosefibers. This reaction may be performed in the presence of at least oneselected from urea and derivatives thereof (hereinafter, referred to as“Compound B”).

One example of the method of allowing Compound A to act on the pulpcomprising cellulose fibers in the presence of Compound B includes amethod of mixing a powder or an aqueous solution of Compound A andCompound B with the pulp comprising cellulose fibers in a dry or wetstate. Another example thereof includes a method of adding a powder oran aqueous solution of Compound A and Compound B to a slurry of the pulpcomprising cellulose fibers. Among them, a method of adding an aqueoussolution of Compound A and Compound B to the pulp comprising cellulosefibers in a dry state, or a method of adding a powder or an aqueoussolution of Compound A and Compound B to the pulp comprising cellulosefibers in a wet state is preferable because of the high homogeneity ofthe reaction. Compound A and Compound B may be added at the same time ormay be added separately. Alternatively, Compound A and

Compound B to be subjected to the reaction may be first added as anaqueous solution, which may be then compressed to squeeze out redundantchemicals. The form of the pulp comprising cellulose fibers ispreferably a cotton-like or thin sheet form, but the form is notparticularly limited thereto.

The phosphorylating agent (Compound A) is at least one selected fromcompounds having phosphoric acid groups and the salts thereof. Examplesof the compound having phosphoric acid groups include, but are notparticularly limited to, phosphoric acid, lithium salts of phosphoricacid, sodium salts of phosphoric acid, potassium salts of phosphoricacid, and ammonium salts of phosphoric acid. Examples of the lithiumsalts of phosphoric acid include lithium dihydrogen phosphate, dilithiumhydrogen phosphate, trilithium phosphate, lithium pyrophosphate, andlithium polyphosphate. Examples of the sodium salts of phosphoric acidinclude sodium dihydrogen phosphate, disodium hydrogen phosphate,trisodium phosphate, sodium pyrophosphate, and sodium polyphosphate.Examples of the potassium salts of phosphoric acid include potassiumdihydrogen phosphate, dipotassium hydrogen phosphate, tripotassiumphosphate, potassium pyrophosphate, and potassium polyphosphate.Examples of the ammonium salts of phosphoric acid include ammoniumdihydrogen phosphate, diammonium hydrogen phosphate, triammoniumphosphate, ammonium pyrophosphate, and ammonium polyphosphate. Amongthese, phosphoric acid, sodium salts of phosphoric acid, potassium saltsof phosphoric acid, and ammonium salts of phosphoric acid are preferablyused.

Since the uniformity of the reaction is improved and the efficiency inintroduction of a phosphoric acid group is further enhanced, thephosphorylating agent (Compound A) is preferably used as an aqueoussolution. Although there is no particular restriction on the pH of anaqueous solution of the phosphorylating agent (Compound A), the pH ispreferably pH 7 or less because the efficiency in introduction ofphosphoric acid groups becomes high, and more preferably pH 3 or moreand pH 7 or less from the viewpoint of suppression of hydrolysis ofcellulose fibers. The pH of an aqueous solution of the Compound A may beadjusted, for example, by using, among compounds having phosphoric acidgroups, a combination of an acidic one and an alkaline one, and changingthe amount ratio thereof. The pH of an aqueous solution of thephosphorylating agent (Compound A) may also be adjusted by adding aninorganic alkali or an organic alkali to an acidic compound amongcompounds having phosphoric acid groups.

The amount of the phosphorylating agent (Compound A) added to the pulpcomprising cellulose fibers is not particularly limited. However, whenthe additive amount of the phosphorylating agent (Compound A) isconverted to the amount of phosphorus atoms, the amount of thephosphorus atoms added to cellulose fibers (absolute dry mass) ispreferably 0.5% by mass or more and 100% by mass or less, morepreferably 1% by mass or more and 50% by mass or less, and mostpreferably 2% by mass or more and 30% by mass or less. If the amount ofthe phosphorus atoms added to cellulose fibers is within theabove-described range, the yield of phosphorylated cellulose fibers canbe further improved. By setting the amount of the phosphorus atoms addedto cellulose fibers at 100% by mass or less, a balance can be keptbetween the effect of improving the yield and costs. On the other hand,by setting the amount of the phosphorus atoms added to cellulose fibersat the above-described lower limit value or more, the yield can beenhanced.

Examples of the Compound B used in the present embodiment include urea,biuret, 1-phenyl urea, 1-benzyl urea, 1-methyl urea, and 1-ethyl urea.

The Compound B is preferably used as an aqueous solution, as with theCompound A. Further, an aqueous solution in which both the Compound Aand Compound B are dissolved is preferably used, because the uniformityof a reaction may be enhanced. The amount of the Compound B added tocellulose fibers (absolute dry mass) is preferably 1% by mass or moreand 500% by mass or less, more preferably 10% by mass or more and 400%by mass or less, further preferably 100% by mass or more and 350% bymass or less, and particularly preferably 150% by mass or more and 300%by mass or less.

The reaction system may contain an amide or an amine, in addition to theCompound A and the Compound B. Examples of the amide include formamide,dimethylformamide, acetamide, and dimethylacetamide. Examples of theamine include methylamine, ethylamine, trimethylamine, triethylamine,monoethanolamine, diethanolamine, triethanolamine, pyridine,ethylenediamine, and hexamethylenediamine. Among them, particularly,triethylamine is known to work as a favorable reaction catalyst.

The phosphoric acid group introduction step preferably has a heatingstep (hereinafter also referred to as a “heat treatment step”). Byestablishing such a heat treatment step, phosphoric acid groups can beefficiently introduced into cellulose fiber.

With regard to the heat treatment temperature applied in the heattreatment step, it is preferable to select a temperature that allows anefficient introduction of phosphoric acid groups while suppressing thethermal decomposition or hydrolysis reaction of cellulose fibers.Specifically, the temperature is preferably 50° C. or higher and 300° C.or lower, more preferably 100° C. or higher and 250° C. or lower, andfurther preferably 130° C. or higher and 200° C. or lower. Moreover, avacuum dryer, an infrared heating device, or a microwave heating devicemay be used for heating.

Upon the heat treatment, if the time for leaving the pulp comprisingcellulose fibers to stand still gets longer while a slurry of the pulpcomprising cellulose fibers to which the Compound A is added containswater, as drying advances, water molecules and the Compound A dissolvedtherein move to the surface of the cellulose fibers. As such, there is apossibility of the occurrence of unevenness in the concentration of theCompound A in the cellulose fibers, and the introduction of phosphoricacid groups into the cellulose fiber surface may not progress uniformly.In order to suppress the occurrence of unevenness in the concentrationof the Compound A in the cellulose fibers due to drying, the pulpcomprising cellulose fibers in the shape of a very thin sheet may beused, or a method of heat-drying or vacuum-drying the pulp comprisingcellulose fibers, while kneading or stirring with the Compound A using akneader or the like, may be employed.

As a heating device used for heat treatment, a device capable of alwaysdischarging moisture retained by slurry or moisture generated by anaddition reaction of phosphoric acid groups with hydroxy groups of thefiber to the outside of the device system is preferable, and forexample, forced convection ovens or the like are preferable. By alwaysdischarging moisture in the device system, in addition to being able tosuppress a hydrolysis reaction of phosphoric acid ester bonds, which isa reverse reaction of the phosphoric acid esterification, acidhydrolysis of sugar chains in the cellulose fibers may be suppressed aswell.

The time for the heat treatment is, although affected by the heatingtemperature, preferably 1 second or more and 300 minutes or less, morepreferably 1 second or more and 1000 seconds or less, and furtherpreferably 10 seconds or more and 800 seconds or less, after water hasbeen substantially removed from the pulp slurry. In the presentinvention, by setting the heating temperature and the heating timewithin an appropriate range, the amount of phosphoric acid groupsintroduced can be set within a preferred range.

The phosphoric acid group introduction step may be performed at leastonce, but may be repeated multiple times as well. This case ispreferable, since more phosphoric acid groups are introduced. Forexample it is also a preferred aspect that the phosphoric acid groupintroduction step is carried out twice.

<Alkali Treatment Step>

After completion of the phosphoric acid group introduction step, it ispreferable to establish an alkali treatment step. The method of alkalitreatment is not particularly limited, but a method of immersing thephosphorylated cellulose fibers in an alkaline solution is applied, forexample.

The alkali compound contained in the alkaline solution is notparticularly limited, but it may be either an inorganic alkalinecompound or an organic alkali compound. The solvent of the alkalinesolution may be either water or an organic solvent. The solvent ispreferably a polar solvent (water, or a polar organic solvent such asalcohol), and the solvent may also be an aqueous solvent. Among alkalinesolutions, a sodium hydroxide aqueous solution, or a potassium hydroxideaqueous solution is particularly preferable, because of highversatility.

The temperature of the alkaline solution in the alkali treatment step isnot particularly limited, but it is preferably 5° C. or higher and 80°C. or lower, and more preferably 10° C. or higher and 60° C. or lower.

The immersion time in the alkaline solution in the alkali treatment stepis not particularly limited, but it is preferably 5 minutes or more and30 minutes or less, and more preferably 10 minutes or more and 20minutes or less.

The amount of the alkaline solution used in the alkali treatment is notparticularly limited, but it is preferably 100% by mass or more and100000% by mass or less, and more preferably 1000% by mass and 10000% bymass or less, with respect to the absolute dry mass of thephosphorylated cellulose fibers.

In order to reduce the amount of an alkaline solution used in the alkalitreatment step, the pulp comprising phosphorylated cellulose fibers maybe washed with water or an organic solvent before the alkali treatmentstep. After the alkali treatment, the alkali-treated pulp comprisingphosphorylated cellulose fibers is preferably washed with water or anorganic solvent in order to improve the handling property.

<Bleaching Step>

After completion of the aforementioned phosphoric acid groupintroduction step or alkali treatment step, a bleaching step isestablished. The bleaching step is a step of bleaching the pulpcomprising cellulose fibers having 0.5 mmol/g or more of phosphoric acidgroups or phosphoric acid group-derived substituents, which is obtainedin the aforementioned phosphoric acid group introduction step.

Since the phosphorylation treatment is generally carried out underweakly acidic conditions and is also attended with a heat treatment at ahigh temperature, it is considered that novel coloration-causingsubstances that are not derived from the raw material pulp are generatedin the phosphorylation treatment process. Accordingly, it is desired tocarry out the bleaching treatment after completion of the phosphoricacid group introduction step.

Examples of the bleaching agent that can be used in the bleaching stepinclude hydrogen peroxide, hydrosulfite, thiourea dioxide, sodiumhypochlorite, and chlorine dioxide. In the bleaching step, the bleachingagent may be used in combination with a bleaching aid, and an example ofsuch a bleaching aid may be a nonionic surfactant.

In the bleaching step, the bleaching agent is preferably added to aslurry, in which the concentration of a pulp comprising phosphorylatedcellulose fibers has been adjusted to 0.2% by mass or more and 20% bymass or less. At this time, the addition percentage of the bleachingagent is preferably 0.1% by mass or more, more preferably 0.5% by massor more, and further preferably 1% by mass or more, with respect to thetotal mass of the phosphorylated cellulose fibers in the slurry. On theother hand, the addition percentage of the bleaching agent is preferably30% by mass or less with respect to the total mass of the phosphorylatedcellulose fibers in the slurry. When the bleaching agent is a chlorinebleaching agent, the bleaching agent is added to the slurry, so that theeffective chlorine concentration in the slurry preferably becomes 50 ppmor more, and more preferably becomes 100 ppm or more. The upper limit ofthe effective chlorine concentration is preferably 2000 ppm. By settingthe amount of the bleaching agent added within the above-describedrange, a pulp comprising cellulose fibers having high whiteness can beobtained.

The bleaching treatment temperature is preferably 10° C. or higher, morepreferably 20° C. or higher, and further preferably 25° C. or higher. Onthe other hand, the bleaching treatment temperature is preferably 150°C. or lower, and more preferably 100° C. or less. By setting thebleaching treatment temperature within the above-described range, a pulpcomprising cellulose fibers having high whiteness can be obtained.

The bleaching treatment time is preferably 10 seconds or more, morepreferably 30 seconds or more, further preferably 1 minute or more, andparticularly preferably 5 minutes or more. On the other hand, thebleaching treatment time is preferably 1 hour or less, and morepreferably 30 minutes or less.

After completion of the bleaching step, a neutralization step may beestablished. In the neutralization step, when a chlorine bleaching agentis particularly used as a bleaching agent, the bleaching agent remainingin the slurry can be neutralized by adding a neutralizing agent such as,for example, sodium thiosulfate or sodium sulfite, so that the bleachingtreatment can be terminated.

After completion of the neutralization step, a washing step ispreferably established. In addition, after completion of theneutralization step, the aforementioned alkali treatment step may beestablished again.

<Defibration Treatment>

The cellulose fibers are ultrafine cellulose fibers having a fiber widthof 1000 nm or less, a defibration treatment step may be establishedafter completion of the bleaching step. In the defibration treatmentstep, fibers are defibrated usually using a defibration treatmentapparatus to yield a slurry comprising ultrafine cellulose fibers, andthere is no particular restriction on a treatment apparatus, or atreatment method.

A high-speed defibrator, a grinder (stone mill-type crusher), ahigh-pressure homogenizer, an ultrahigh-pressure homogenizer, ahigh-pressure collision-type crusher, a ball mill, a bead mill, or thelike can be used as the defibration treatment apparatus. Alternatively,for example, a wet milling apparatus such as a disc-type refiner, aconical refiner, a twin-screw kneader, an oscillation mill, a homomixerunder high-speed rotation, an ultrasonic disperser, or a beater may alsobe used as the defibration treatment apparatus. The defibrationtreatment apparatus is not limited to the above. Examples of a preferreddefibration treatment method include a high-speed defibrator, ahigh-pressure homogenizer, and an ultrahigh-pressure homogenizer, whichare less affected by milling media, and are free from apprehension ofcontamination.

Upon the defibration treatment, the fiber raw material is preferablydiluted with water and an organic solvent each alone or in combination,to prepare a slurry, though the method is not particularly limitedthereto. Water as well as a polar organic solvent can be used as adispersion medium. Preferred examples of the polar organic solventinclude, but are not particularly limited to, alcohols, ketones, ethers,dimethyl sulfoxide (DMSO), dimethylformamide (DMF), anddimethylacetamide (DMAc). Examples of the alcohols include methanol,ethanol, n-propanol, isopropanol, n-butanol, and t-butyl alcohol.Examples of the ketones include acetone and methyl ethyl ketone (MEK).Examples of the ethers include diethyl ether and tetrahydrofuran (THF).One of these dispersion media may be used, or two or more thereof may beused. The dispersion medium may also contain a solid content other thanthe fiber raw material, for example, hydrogen-binding urea.

With regard to the ultrafine cellulose fibers, the ultrafine cellulosefiber-containing slurry obtained by the defibration treatment may beonce concentrated and/or dried, and then, may be subjected to adefibration treatment again. In this case, there is no particularrestriction on the method of concentration and drying, but examplesthereof include a method in which a concentrating agent is added into aslurry comprising ultrafine cellulose fibers, and a method using adehydrator, a press, a dryer, and the like used generally. Further,publicly known methods, for example as described in WO 2014/024876, WO2012/107642, and WO 2013/121086, may be used. Also, the ultrafinecellulose fiber-containing slurry may be formed into a sheet, so that itis concentrated and dried. The formed sheet is subjected to adefibration treatment, so that an ultrafine cellulose fiber-containingslurry can be obtained again.

Examples of a device used for defibrating (pulverizing) the ultrafinecellulose fiber-containing slurry again, after the concentration and/ordrying of the ultrafine cellulose fiber-containing slurry, include, butare not particularly limited to, a high-speed defibrator, a grinder(stone mill-type grinder), a high-pressure homogenizer, an ultra-highpressure homogenizer, a high-pressure collision type crusher, a ballmill, a bead mill, a disk type refiner, a conical refiner, a twin screwkneader, a vibrating mill, and a device for wet milling, such as ahigh-speed rotating homomixer, an ultrasonic disperser, or a beater.

(Physical Properties of Cellulose Fibers (Pulp) Before and AfterBleaching)

The method for producing a pulp comprising cellulose fibers of thepresent invention is also characterized in that various types ofphysical properties of cellulose fibers or a pulp comprising thecellulose fibers are not largely changed before and after the bleachingstep. Specifically, in the method for producing a pulp comprisingcellulose fibers of the present invention, even in the case of bleachinga pulp comprising cellulose fibers having 0.5 mmol/g or more ofphosphoric acid groups or phosphoric acid group-derived substituents,the cellulose fibers or the pulp comprising the cellulose fibers canmaintain the physical properties or characteristics thereof.

In the present invention, a difference between the amount of phosphoricacid groups in cellulose fibers before the bleaching step and the amountof phosphoric acid groups in cellulose fibers after the bleaching stepis preferably 0.2 mmol/g or less, more preferably 0.1 mmol/g or less,and further preferably 0.05 mmol/g or less.

Moreover, in the present invention, ultrafine cellulose fibers capableof setting a reduction in the polymerization degree within a desiredrange can be obtained from a pulp comprising cellulose fibers.Specifically, a difference between the viscosity average polymerizationdegree of ultrafine cellulose fibers obtained by subjecting a pulpcomprising cellulose fibers before the bleaching step to a fibrillationtreatment performed under the following condition e, and the viscosityaverage polymerization degree of ultrafine cellulose fibers obtained bysubjecting a pulp comprising cellulose fibers after the bleaching stepto a fibrillation treatment performed under the following condition e,is preferably 100 or less, more preferably 50 or less, and furtherpreferably 30 or less.

(Condition e)

A pulp comprising cellulose fibers is diluted with ion exchange water toa concentration of 0.5% by mass, so as to obtain a slurry, and theslurry is then subjected to a fibrillation treatment using CLEARMIX-2.2Smanufactured by M Technique Co., Ltd., at a rotation number of 21500 rpmfor 30 minutes.

The viscosity average polymerization degree of ultrafine cellulosefibers obtained by performing a fibrillation treatment under theabove-described condition e is a value calculated from the viscosity ofa pulp measured in accordance with Tappi T230. Specifically, theultrafine cellulose fibers are dispersed in a dispersion medium, theviscosity thereof is then measured (defined as 111), and the viscosityof only the dispersion medium is then measured (defined as η0).Thereafter, a specific viscosity (ηsp) and an intrinsic viscosity ([η1])are calculated according to the following equations.

ηsp=(η1/η0)−1

[η]=ηsp/(c(1+0.28×ηsp))

In the above equation, c indicates the concentration of ultrafinecellulose fibers upon the measurement of the viscosity.

Further, polymerization degree (DP) is calculated according to thefollowing equation.

SP=1.75×[η]

Since this polymerization degree is an average polymerization degreemeasured according to a viscosity method, it is referred to as a“viscosity average polymerization degree.”

Furthermore, in the present invention, ultrafine cellulose fiberscapable of exhibiting a desired viscosity can be obtained from a pulpcomprising cellulose fibers. Specifically, a pulp comprising cellulosefibers before the bleaching step is subjected to a fibrillationtreatment under the condition e to obtain an ultrafine cellulosefiber-containing slurry 1 having a concentration of 0.4% by mass. Theviscosity of this slurry 1 is defined as P. On the other hand, a pulpcomprising cellulose fibers after the bleaching step is subjected to afibrillation treatment under the condition e to obtain an ultrafinecellulose fiber-containing slurry 2 having a concentration of 0.4% bymass. The viscosity of this slurry 2 is defined as Q. The P/Q value ispreferably 0.5 or more and 2.0 or less. The P/Q value is more preferably0.5 or more and 1.5 or less, and further preferably 0.5 or more and 1.2or less.

It is to be noted that the viscosity of such an ultrafine cellulosefiber-containing slurry is obtained by homogeneously stirring a slurryusing a disperser at 1500 rpm, then leaving the obtained slurry at restfor 24 hours, and then measuring the viscosity of the resulting slurryusing a type B viscometer (manufactured by BROOKFIELD; analog viscometerT-LVT). Regarding measurement conditions, the slurry is rotated at 25°C. at 3 rpm for 3 minutes, and then, the viscosity is measured.

(Sheet)

The present invention also relates to a sheet produced using the pulpcomprising cellulose fibers as mentioned above in the presentdescription. The cellulose fibers contained in the sheet of the presentinvention are, for example, ultrafine cellulose fibers. In the presentdescription, the sheet containing ultrafine cellulose fibers is alsoreferred to as an “ultrafine cellulose fiber-containing sheet.” Thesheet of the present invention relates to a sheet containingphosphorylated ultrafine cellulose fibers having 0.5 mmol/g or more ofphosphoric acid groups or phosphoric acid group-derived substituents andalso having a fiber width of 1000 nm or less.

In the sheet of the present invention, YI₃₀ calculated by the followingEquation d is 0.57 or less. YI₃₀ is preferably 0.55 or less, morepreferably 0.50 or less, and further preferably 0.45 or less.

Yellowness (YI₃₀) of sheet relative to film thickness of 30μm=yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm))  (d)

In the above equation, the yellowness (YI) of a sheet is measured inaccordance with JIS K 7373.

(Method for Producing Sheet)

The sheet of the present invention can be produced, for example, byfiltrating a composition comprising ultrafine cellulose fibers (e.g., asuspension or a slurry) to form a wet sheet on the filter. The formedwet sheet is peeled off from the filter, and is then dried on astainless steel tray or the like, so that a dried sheet can be obtained.

Alternatively, the sheet of the present invention can be produced byapplying a composition comprising ultrafine cellulose fibers (e.g., asuspension or a slurry) onto a base material, or by papermaking from acomposition comprising ultrafine cellulose fibers (e.g., a suspension ora slurry).

<Coating Step>

The coating step is a step of applying a composition (slurry, etc.) ontoa base material, and then drying the composition to form a sheet. Use ofa coating apparatus and a long base material can continuously producesheets. When a slurry is applied onto a base material, the concentrationof the slurry to be applied is not particularly limited, but it ispreferably 0.05% by mass or more and 5% by mass or less.

The material of the base material used in the coating step is notparticularly limited. Although a base material having higher wettabilityto the composition (slurry) is preferable because shrinkage of the sheetor the like upon drying is suppressed, it is preferable to select onefrom which a sheet formed after drying can be easily detached. Of these,a resin film or plate, or a metal film or plate is preferable, but isnot particularly limited thereto. Examples of the base material that canbe used herein include: resin films or plates, such as those made ofacrylic acid, polyethylene terephthalate, vinyl chloride, polystyrene,or polyvinylidene chloride; metal films or plates, such as those made ofaluminum, zinc, copper, or iron; these films or plates obtained by theoxidation treatment of surface thereof; and stainless films or platesand brass films or plates.

When the composition (slurry) has a low viscosity and spreads on thebase material in the coating step, a damming frame may be fixed and usedon the base material in order to obtain an ultrafine cellulosefiber-containing sheet having a predetermined thickness and basisweight. The material of the damming frame is not particularly limited,but it is preferable to select ones from which edges of the sheet adhereafter drying can be easily detached. Of these, frames formed from resinfilms or plates or metal films or plates are preferable, but are notparticularly limited thereto. Example thereof that can be used hereininclude frames formed from resin films or plates, such as those made ofacrylic acid, polyethylene terephthalate, vinyl chloride, polystyrene,or polyvinylidene chloride; from metal films or plates, such as thosemade of aluminum, zinc, copper, or iron; from these films or platesobtained by the oxidation treatment of surface thereof; and fromstainless films or plates and brass films or plates.

Examples of a coater for applying the composition (slurry) that can beused herein include roll coaters, gravure coaters, die coaters, curtaincoaters, and air doctor coaters. die coaters, curtain coaters, and spraycoaters are preferable because more even thickness can be provided.

The coating temperature is not particularly limited, but it ispreferably 20° C. or higher and 45° C. or lower, more preferably 25° C.or higher and 40° C. or lower, and further preferably 27° C. or higherand 35° C. or lower. When the coating temperature is equal to or higherthan the above-described lower limit value, it is possible to easilyapply the composition (slurry). When the coating temperature is equal toor lower than the above-described upper limit value, it is possible tosuppress volatilization of the dispersion medium upon coating.

In the coating step, it is preferable to apply the slurry so as toachieve a finished basis weight of the sheet that is 10 g/m² or more and100 g/m² or less. By applying the slurry so as to achieve a basis weightthat is within the above-described range, a sheet having excellentstrength can be obtained.

The step of forming a sheet preferably includes a step of drying thecomposition (slurry) applied onto the base material. The drying methodis not particularly limited, but any of a contactless drying method anda method of drying the sheet while locking the sheet may be used, orthese methods may also be used in combination.

The contactless drying method is not particularly limited, but a methodfor drying by heating with hot air, infrared radiation, far-infraredradiation, or near-infrared radiation (a drying method by heating) or amethod for drying in vacuum (a vacuum drying method) can be utilized.Although the drying method by heating and the vacuum drying method maybe combined, the drying method by heating is usually utilized. Thedrying with infrared radiation, far-infrared radiation, or near-infraredradiation can be performed using an infrared apparatus, a far-infraredapparatus, or a near-infrared apparatus without particular limitations.The heating temperature for the drying method by heating is notparticularly limited, but it is preferably 20° C. or higher and 120° C.or lower, and more preferably 25° C. or higher and 105° C. or lower. Atthe heating temperature equal to or higher than the above-describedlower limit value, the dispersion medium can be rapidly volatilized. Atthe heating temperature equal to or lower than the above-described upperlimit value, cost required for the heating can be reduced, and thethermal discoloration of the ultrafine cellulose fibers can besuppressed.

The sheet may be peeled off from the base material, and may be thenwound up. Alternatively, a laminate of the sheet and the base materialmay be directly wound up, and the sheet may be then peeled off from thebase material immediately before the use thereof. Otherwise, a laminatecomprising a base material as a portion thereof may be used withoutpeeling the base material off.

<Papermaking Step>

The step of forming a sheet may include a step of papermaking from acomposition (slurry). Examples of a paper machine used in thepapermaking step include continuous paper machines such as a Fourdrinierpaper machine, a cylinder paper machine, and an inclined paper machine,and a multilayer combination paper machine, which is a combinationthereof Known papermaking such as papermaking by hand may be carried outin the papermaking step.

In the papermaking step, the composition (slurry) is wire-filtered anddehydrated to obtain a sheet that is in a wet state. The sheet is thenpressed and dried to obtain a sheet. The concentration of the slurry isnot particularly limited, but it is preferably 0.05% by mass or more and5% by mass or less. Upon filtration and dehydration of the slurry, afilter fabric for filtration is not particularly limited. It isimportant that ultrafine cellulose fibers do not pass through the filterfabric and the filtration speed is not excessively slow. Such filterfabric is not particularly limited, and a sheet consisting of an organicpolymer, a woven fabric, or a porous membrane is preferable. Preferredexamples of the organic polymer include, but are not particularlylimited to, non-cellulose organic polymers such as polyethyleneterephthalate, polyethylene, polypropylene, and polytetrafluoroethylene(PTFE). Specific examples thereof include, but are not particularlylimited to, a polytetrafluoroethylene porous membrane having a pore sizeof 0.1 μm or more and 20 μm or less, for example, 1 μm, and woven fabricmade of polyethylene terephthalate or polyethylene having a pore size of0.1 μm or more and 20 μm or less, for example, 1 μm.

A method for producing a sheet from a composition (slurry) is notparticularly limited, but an example thereof is the method disclosed inWO 2011/013567 comprising using a production apparatus. This productionapparatus comprises a dewatering section for ejecting an ultrafinecellulose fiber-containing slurry onto the upper surface of an endlessbelt and then dewatering a dispersion medium contained in the ejectedslurry to form a web, and a drying section for drying the web to producea fiber sheet. The endless belt is provided across from the dewateringsection to the drying section, and the web formed in the dewateringsection is transferred to the drying section while being placed on theendless belt.

The dehydration method that can be used in the present invention is notparticularly limited. An example of the method is a dehydration methodconventionally used for paper production. A preferred example is amethod comprising performing dehydration using a Fourdrinier, cylinder,tilted wire, or the like and then performing dehydration using a rollpress. In addition, a drying method is not particularly limited, but anexample thereof is a method used for paper production and for example amethod using a cylinder dryer, a yankee dryer, hot air drying, anear-infrared heater, or an infrared heater is preferable.

(Laminate)

In the present invention, a laminate having the aforementioned sheet anda resin layer that is disposed on at least one surface of the sheet maybe produced.

The resin layer is a layer that has a natural resin or a synthetic resinas a main component. In this context, the main component refers to acomponent comprised in 50% by mass or more, based on the total mass ofthe resin layer. The content of the resin is preferably 60% by mass ormore, more preferably 70% by mass or more, further preferably 80% bymass or more, and particularly preferably 90% by mass or more, based onthe total mass of the resin layer. It is to be noted that the content ofthe resin may be set at 100% by mass, or may also be set at 95% by massor less.

Examples of natural resins may include rosin-based resins, such asrosin, rosin ester and hydrated rosin ester.

The synthetic resin is preferably at least one selected from, forexample, polycarbonate resins, polyethylene terephthalate resins,polyethylene naphthalate resins, polyethylene resins, polypropyleneresins, polyimide resins, polystyrene resins, acrylic resins, epoxyresins, urethane resins, fluorine resins, and silicon resins. Amongthem, the synthetic resin is preferably a polycarbonate resin, anacrylic resin, or a silicon resin.

Moreover, the synthetic resin may also be an adhesive that constitutesan adhesive layer. Examples of such a synthetic resin include acrylicresins, vinyl chloride resins, (meth)acrylic acid ester resins,styrene/acrylic acid ester copolymer resins, vinyl acetate resins, vinylacetate/(meth)acrylic acid ester copolymer resins, urethane resins,silicone resins, epoxy resins, ethylene/vinyl acetate copolymer resins,polyester-based resins, polyvinyl alcohol resins, ethylene vinyl alcoholcopolymer resins, and rubber-based emulsions such as SBR and NBR.

One resin that constitutes the resin layer may be used alone, or acopolymer obtained by copolymerization or graft polymerization of aplurality of resin components may be used. Alternatively, a plurality ofresin components may be mixed by a physical process and used as a blendmaterial.

As a resin layer established on each surface side of the sheet, a singleresin layer may be established, or multiple resin layers may also beestablished. In the case of establishing multiple resin layers, theresin layers each comprising the aforementioned adhesive constituting anadhesive layer and at least one selected from a polycarbonate resin, anacrylic resin and a silicon resin may be formed.

Upon the production of a laminate, a laminate may be formed by applyinga resin composition for use in the formation of a resin layer on asheet. Otherwise, the previously formed resin layer may be laminated ona sheet. In this case, an adhesive layer may be established between theresin layer and the sheet, and such an adhesive layer is included in theresin layer. Moreover, when the base material used in the production ofa sheet is a resin, the base material may not be peeled off, and may beused as a portion of the resin layer.

(Intended Use)

The pulp comprising cellulose fibers of the present invention is used invarious forms. For example, the pulp comprising cellulose fibers of thepresent invention may be processed into a form such as a slurry or asolid. When the pulp comprising cellulose fibers of the presentinvention is a solid, a sheet or a particulate may be formed from thepulp comprising cellulose fibers. Since such a sheet or a particulatehas high whiteness, they are preferably used in intended uses requiredto have high whiteness. For example, from the viewpoint of utilizingsuch properties as high whiteness, it is suitable for the pulpcomprising cellulose fibers of the present invention to be used inabsorbing materials such as diapers or sanitary goods, which arerequired to have apparent cleanliness, and in dust filters, which arealso required to have cleanliness.

Moreover, the amount of phosphoric acid groups introduced into theaforementioned cellulose fibers after the bleaching step is at the samelevel as that of the amount of phosphoric acid groups introduced intocellulose fibers before the bleaching step, and thus, the amount ofphosphoric acid groups introduced is sufficient. Accordingly, theaforementioned cellulose fibers can be easily fibrillated, and thus, aphosphorylated ultrafine cellulose fiber-containing slurry or aphosphorylated ultrafine cellulose fiber-containing sheet hassufficiently high transparency. That is, sufficiently fibrillatedphosphorylated ultrafine cellulose fibers with less colored can beobtained. From the viewpoint of utilizing the aforementioned properties,the phosphorylated ultrafine cellulose fibers are suitable for intendeduses, such as light transmissible substrates for various displaydevices, various solar cells, and the like. In addition, thephosphorylated ultrafine cellulose fibers are also suitable for intendeduses, such as substrates of electronic devices, components of consumerelectronics, window materials of various types of vehicles or buildings,interior materials, exterior materials, and wrapping materials.Moreover, the phosphorylated ultrafine cellulose fibers are alsosuitable for intended uses, such as threads, filters, woven fabrics,buffering materials, sponges, and polishing materials, and also, otherintended uses, in which the sheet itself is used as a reinforcingmaterial.

EXAMPLES

The characteristics of the present invention will be more specificallydescribed in the following examples and comparative examples. Thematerials, used amounts, ratios, treatment contents, treatmentprocedures, etc. described in the following examples can beappropriately modified, unless they are deviated from the gist of thepresent invention. Accordingly, the scope of the present inventionshould not be restrictively interpreted by the following specificexamples.

Example 1 <Phosphorylation Reaction Step>

Pulp manufactured by Oji Paper Co., Ltd. (solid content: 93% by mass,basis weight: 208 g/m², sheet-shaped, Canadian Standard Freeness (CSF)measured according to JIS P 8121 after disintegration: 700 ml), whichwas needle bleached kraft pulp, was used as a raw material. A mixedaqueous solution of ammonium dihydrogen phosphate and urea was added to100 parts by mass of the needle bleached kraft pulp (absolute dry mass),and the obtained mixture was then compressed to result in 45 parts bymass of the ammonium dihydrogen phosphate, 120 parts by mass of theurea, and 150 parts by mass of ion exchange water, so as to obtain achemical-impregnated pulp. The obtained chemical-impregnated pulp wassubjected to drying and heating treatments in a hot-air dryer of 165° C.for 200 seconds, so that phosphoric acid groups were introduced intocellulose in the pulp, thereby obtaining a pulp comprisingphosphorylated cellulose fibers A.

<Washing and Alkali Treatment Step>

Ion exchange water was poured onto the obtained pulp comprisingphosphorylated cellulose fibers A, followed by stiffing the obtainedmixture for uniform dispersion. Thereafter, the reaction mixture wassubjected to filtration and dehydration to obtain a dehydrated sheet. Byrepeating this operation, redundant chemical liquid was fully washedaway. Subsequently, the resultant was diluted with ion exchange water,so that the concentration of the pulp comprising phosphorylatedcellulose fibers became 2% by mass, and then, a 1 N sodium hydroxideaqueous solution was gradually added to the resultant, while stirring,so as to obtain a slurry with pH 12±0.2. Thereafter, this slurry wasdehydrated to obtain a dehydrated sheet, and ion exchange water was thenpoured onto the dehydrated sheet again, followed by stiffing theobtained mixture for uniform dispersion. After that, the reactionmixture was subjected to filtration and dehydration to obtain adehydrated sheet. By repeating this operation, redundant sodiumhydroxide was fully washed away, so as to obtain a pulp comprisingphosphorylated cellulose fibers B.

<Bleaching and Alkali Treatment Step>

Ion exchange water was poured onto the obtained pulp comprisingphosphorylated cellulose fibers B, so that the concentration of the pulpcomprising phosphorylated cellulose fibers became 2% by mass, and theobtained mixture was homogeneously stirred to obtain 499 g of asuspension. To the suspension, 5 mL of sodium hypochlorite having aneffective chlorine concentration of 53.8 g/L was added, so that theeffective chlorine concentration in the suspension became 549 ppm, andwhile stirring, a bleaching treatment was initiated at room temperature.It is to be noted that the additive amount of sodium hypochlorite was2.7% by mass with respect to the mass of the pulp comprisingphosphorylated cellulose fibers. After the bleaching treatment had beenperformed for 5 minutes, 0.1 N sodium thiosulfate was added to thesuspension in an amount sufficient for converting the effective chlorineconcentration in the suspension to 0 (zero), so that the effectivechlorine remaining in the suspension was neutralized and the bleachingtreatment was thereby terminated. By repeating the operation ofperforming filtration and dehydration on the bleached suspension toobtain a dehydrated sheet, remaining ions such as redundant sodiumthiosulfate were fully washed away. Subsequently, the resultant wasdiluted with ion exchange water, so that the concentration of the pulpcomprising phosphorylated cellulose fibers became 2% by mass, and whilestirring, a 1 N sodium hydroxide aqueous solution was gradually added tothe solution to obtain a pulp slurry with pH 12±0.2. Thereafter, thispulp slurry was dehydrated to obtain a dehydrated sheet, and ionexchange water was poured on the sheet again, followed by stirring foruniform dispersion. Then, the resulting solution was subjected tofiltration and dehydration to obtain a dehydrated sheet. By repeatingthis operation of obtaining a dehydrated sheet, redundant sodiumhydroxide was fully washed away, so as to obtain a pulp comprisingbleached phosphorylated cellulose fibers C. The ISO whiteness, hue, andpolymerization degree of the pulp comprising bleached phosphorylatedcellulose fibers C were measured according to the after-mentionedmethods.

<Mechanical Treatment>

Ion exchange water was added to the pulp comprising bleachedphosphorylated cellulose fibers C, so as to prepare a suspension, inwhich the concentration of the pulp comprising bleached phosphorylatedcellulose fibers was 0.5% by mass. This suspension was subjected to afibrillation treatment (defibration treatment), using a defibrationtreatment device (manufactured by M Technique Co., Ltd., CLEARMIX-2.2S)under conditions of 21500 rpm for 30 minutes, to obtain an ultrafinecellulose fiber-containing slurry. The obtained ultrafine cellulosefiber-containing slurry was measured in terms of supernatant yield,viscosity, haze, and the amount of phosphoric acid groups containedtherein, according to the after-mentioned methods.

Example 2

A pulp comprising phosphorylated cellulose fibers B and a pulpcomprising bleached phosphorylated cellulose fibers C were obtained inthe same manner as that of Example 1, with the exception that 1 mL ofsodium hypochlorite having an effective chlorine concentration of 53.8g/L was added, so that the effective chlorine concentration in thesuspension became 110 ppm in the <Bleaching and alkali treatment step>of Example 1.

Example 3

A pulp comprising phosphorylated cellulose fibers B and a pulpcomprising bleached phosphorylated cellulose fibers C were obtained inthe same manner as that of Example 1, with the exception that 10 mL ofsodium hypochlorite having an effective chlorine concentration of 53.8g/L was added, so that the effective chlorine concentration in thesuspension became 1097 ppm in the <Bleaching and alkali treatment step >of Example 1.

Example 4

A pulp comprising phosphorylated cellulose fibers B, a pulp comprisingbleached phosphorylated cellulose fibers C, and an ultrafine cellulosefiber-containing slurry were obtained in the same manner as that ofExample 1, with the exception that the bleaching treatment time was setat 1 minute in the <Bleaching and alkali treatment step> of Example 1.

Example 5

A pulp comprising phosphorylated cellulose fibers B, a pulp comprisingbleached phosphorylated cellulose fibers C, and an ultrafine cellulosefiber-containing slurry were obtained in the same manner as that ofExample 1, with the exception that the bleaching treatment time was setat 15 minutes in the <Bleaching and alkali treatment step> of Example 1.

Example 6

A pulp comprising phosphorylated cellulose fibers B and a pulpcomprising bleached phosphorylated cellulose fibers C were obtained inthe same manner as that of Example 1, with the exception that, in the<Bleaching and alkali treatment step> of Example 1, 1 mL of sodiumhypochlorite having an effective chlorine concentration of 53.8 g/L wasadded, so that the effective chlorine concentration in the suspensionbecame 110 ppm, and the bleaching treatment was carried out in a hotwater bath at 40° C.

Example 7 <Phosphoric Acid Group Introduction Step (Second Time)>

The <Phosphorylation reaction step >and <Washing and alkali treatmentstep> were carried out in the same manner as that of Example 1 to obtaina pulp comprising phosphorylated cellulose fibers B1. Using the obtainedpulp comprising phosphorylated cellulose fibers B1 as a raw material,the aforementioned <Phosphorylation reaction step> was carried outthereon again, and phosphoric acid groups were further introduced intothe cellulose in the resulting cellulose fibers, so as to obtain a pulpcomprising phosphorylated cellulose fibers B2.

<Washing and Alkali Treatment Step (Second Time)>

The aforementioned <Washing and alkali treatment step >was carried outon the obtained pulp comprising phosphorylated cellulose fibers B2 toobtain a pulp comprising phosphorylated cellulose fibers B3.

<Bleaching and Alkali Treatment Step > and <Mechanical Treatment>

The <Bleaching and alkali treatment step> and <Mechanical treatment>were carried out in the same manner as that of Example 1, with theexception that, in the <Bleaching and alkali treatment step> of Example1, 10 mL of sodium hypochlorite having an effective chlorineconcentration of 53.8 g/L was added, so that the effective chlorineconcentration in the suspension became 1097 ppm, and the bleachingtreatment time was set at 15 minutes, thereby obtaining a pulpcomprising bleached phosphorylated cellulose fibers C, and an ultrafinecellulose fiber-containing slurry.

Example 8 <Phosphorylation Reaction Step> and <Washing And AlkaliTreatment Step>

The <Phosphorylation reaction step >and <Washing and alkali treatmentstep> were carried out in the same manner as that of Example 1 to obtaina pulp comprising phosphorylated cellulose fibers B.

<Bleaching and Alkali Treatment Step>

Ion exchange water was poured onto the obtained pulp comprisingphosphorylated cellulose fibers B, so that the concentration of the pulpcomprising phosphorylated cellulose fibers became 2% by mass, and theobtained mixture was homogeneously stirred to obtain 447 g of asuspension. To the suspension, 52.8 mL of chlorine dioxide having aneffective chlorine concentration of 5.1 g/L was added, so that theeffective chlorine concentration in the suspension became 549 ppm, andthen, the obtained mixture was fully kneaded, so that chlorine dioxidewas fully dispersed in the suspension. Thereafter, a bleaching treatmentwas initiated in a hot water bath at 70° C. It is to be noted that theadditive amount of chlorine dioxide was 2.7% by mass with respect to themass of the phosphorylated cellulose fibers. After the bleachingtreatment had been performed for 5 minutes, 0.1 N sodium thiosulfate wasadded to the suspension in an amount sufficient for neutralizingchlorine dioxide, so that the effective chlorine remaining in thesuspension was neutralized and the bleaching treatment was therebyterminated. By repeating the operation of performing filtration anddehydration on the bleached suspension to obtain a dehydrated sheet,remaining ions such as redundant sodium thiosulfate were fully washedaway. Subsequently, the resultant was diluted with ion exchange water,so that the concentration of the pulp comprising phosphorylatedcellulose fibers became 2% by mass, and while stiffing, a 1 N sodiumhydroxide aqueous solution was gradually added to the solution to obtaina slurry with pH 12±0.2. Thereafter, this slurry was dehydrated toobtain a dehydrated sheet, and ion exchange water was poured on thesheet again, followed by stirring for uniform dispersion. Then, theresulting solution was subjected to filtration and dehydration to obtaina dehydrated sheet. By repeating this operation of obtaining adehydrated sheet, redundant sodium hydroxide was fully washed away, soas to obtain a pulp comprising bleached phosphorylated cellulose fibersC.

Example 9 <Phosphorylation Reaction Step > and <Washing And AlkaliTreatment Step>

The <Phosphorylation reaction step > and <Washing and alkali treatmentstep> were carried out in the same manner as that of Example 1 to obtaina pulp comprising phosphorylated cellulose fibers B.

<Bleaching and Alkali Treatment Step>

Ion exchange water was poured onto the obtained pulp comprisingphosphorylated cellulose fibers B, so that the concentration of the pulpcomprising phosphorylated cellulose fibers became 2% by mass, and theobtained mixture was homogeneously stiffed to obtain 499 g of asuspension. To the suspension, 1.5 g of thiourea dioxide (FAS) wasadded, so that the addition percentage of thiourea dioxide (FAS) became15% by mass with respect to the mass of the pulp comprisingphosphorylated cellulose fibers in the suspension, and thereafter, ableaching treatment was initiated while stirring in a hot water bath at80° C. After the bleaching treatment had been performed for 15 minutes,the operation of performing filtration and dehydration on the bleachedsuspension to obtain a dehydrated sheet was repeatedly carried out, sothat remaining ions such as redundant sodium thiosulfate were fullywashed away. Subsequently, the resultant was diluted with ion exchangewater, so that the concentration of the pulp comprising phosphorylatedcellulose fibers became 2% by mass, and while stiffing, a 1 N sodiumhydroxide aqueous solution was gradually added to the solution to obtaina slurry with pH 12 ±0.2. Thereafter, this slurry was dehydrated toobtain a dehydrated sheet, and ion exchange water was poured on thesheet again, followed by stirring for uniform dispersion. Then, theresulting solution was subjected to filtration and dehydration to obtaina dehydrated sheet. By repeating this operation of obtaining adehydrated sheet, redundant sodium hydroxide was fully washed away, soas to obtain a pulp comprising bleached phosphorylated cellulose fibersC.

Comparative Example 1

A pulp comprising phosphorylated cellulose fibers B and an ultrafinecellulose fiber-containing slurry were obtained in the same manner asthat of Example 1, with the exception that the <Bleaching and alkalitreatment step> was not carried out in Example 1.

Comparative Example 2

A pulp comprising phosphorylated cellulose fibers B and an ultrafinecellulose fiber-containing slurry were obtained in the same manner asthat of Example 7, with the exception that the <Bleaching and alkalitreatment step > was not carried out in Example 7.

(Analysis and Evaluation) <Measurement of ISO Whiteness and b* Value ofPulp Comprising Phosphorylated Cellulose Fibers>

Ion exchange water was added to the pulp comprising bleachedphosphorylated cellulose fibers obtained in the aforementioned Examples,or the pulp comprising phosphorylated cellulose fibers obtained in theaforementioned Comparative Examples, so as to prepare a suspension, inwhich the concentration of the pulp comprising phosphorylated cellulosefibers was 0.3% by mass, and the obtained mixture was then stirred toobtain a fully homogeneous suspension. This suspension was filtratedusing Separato, so that a wet sheet having an absolute dry basis weightof 200 g/m² was formed on a filter paper (manufactured by Advantec ToyoKaisha, Ltd., φ 90 mm). The wet sheet was peeled off from the filterpaper, was then placed on a stainless steel tray, and was then driedunder conditions of 23° C. and a relative humidity of 50% for 3 days.Both surfaces of the dried sheet were sandwiched by papers and metalplates, and thereafter, using mini-hot press (manufactured by Toyo SeikiKogyo Co., Ltd., MP-SNH), the sheet was pressed by a pressure of 7.7 MPafor 1 minute, to obtain a pulp sheet. It is to be noted that four pulpsheets were produced in each of the Examples and Comparative Examples.The four pulp sheets were laminated on one another, and using awhiteness spectrophotometer (manufactured by Suga Test Instruments Co.,Ltd., SC-10WN), ISO whiteness (in accordance with JIS P 8148) and b*value (in accordance with JIS P 8150) were measured.

TABLE 1 Bleaching treatment conditions Phosphorylated cellulose Drug-Effective Bleaching Bleaching fiber-containing sheet Number of Pulpadding chlorine treatment treatment ISO phosphor- concentration rateconcentration temperature time whiteness ylations Drug [mass %] [mass %][ppm] [° C.] [min] [%] b* Ex. 1 Once Sodium hypochlorite 2 2.7 549 25 584.92 3.56 Ex. 2 Once Sodium hypochlorite 2 0.5 110 25 5 83.63 4.48 Ex.3 Once Sodium hypochlorite 2 5.4 1097 25 5 86.34 2.84 Ex. 4 Once Sodiumhypochlorite 2 2.7 549 25 1 84.79 3.70 Ex. 5 Once Sodium hypochlorite 22.7 549 25 15 85.69 3.07 Ex. 6 Once Sodium hypochlorite 2 0.5 110 40 584.42 3.98 Ex. 7 Twice Sodium hypochlorite 2 5.4 1097 25 15 85.25 2.92Ex. 8 Once Chlorine dioxide 2 2.7 549 70 5 85.86 3.04 Ex. 9 OnceThiourea dioxide 2 15 — 80 15 83.14 4.59 Comp. Once — 2 — — — — 80.056.06 Ex. 1 Comp. Twice — 2 — — — — 78.44 5.91 Ex. 2

In Example 1, 4, 5 and 7, and Comparative Examples 1 and 2, theaforementioned <Mechanical treatment >was carried out to obtainultrafine cellulose fibers. The obtained ultrafine cellulosefiber-containing slurry and ultrafine cellulose fibers were measured asfollows.

<Measurement of Supernatant Yield>

The yield of a supernatant obtained after centrifugation of theultrafine cellulose fiber-containing slurry was measured according tothe following method. The supernatant yield obtained aftercentrifugation serves as an indicator of the yield of ultrafinecellulose fibers. The higher the supernatant yield, the higher the yieldof ultrafine cellulose fibers that can be obtained.

Ion exchange water was added to the ultrafine cellulose fiber-containingslurry to obtain a slurry having a solid concentration of 0.1% by mass(Slurry A). The Slurry A was centrifuged using a cooled high-speedcentrifugal separator (manufactured by KOKUSAN Co. Ltd., H-2000B) underconditions of 12000 G for 10 minutes. The obtained supernatant (referredto as “Slurry B”) was recovered, and the solid concentration in thesupernatant was then measured. After that, the supernatant yield (theyield of ultrafine cellulose fibers) was calculated according to thefollowing equation:

Supernatant yield (%)=solid concentration (% by mass) in Slurry B/solidconcentration (% by mass) in Slurry A×100.

<Measurement of Polymerization Degree>

The polymerization degree of the ultrafine cellulose fibers wascalculated from the viscosity of a pulp measured in accordance withTappi T230. Specifically, the ultrafine cellulose fibers as ameasurement target were dispersed in a dispersion medium, the viscositythereof was then measured (defined as η1), and the blank viscosity wasthen measured using only the dispersion medium (defined as η0).Thereafter, a specific viscosity (ηsp) and an intrinsic viscosity ([η])were calculated according to the following equations.

ηsp=(η1/η0)−1

[η]=ηsp/(c(1+0.28×ηsp))

In the above equation, c indicates the concentration of ultrafinecellulose fibers upon the measurement of the viscosity.

Further, the polymerization degree (DP) was calculated according to thefollowing equation.

DP=1.75×[η]

Since this polymerization degree is an average polymerization degreemeasured according to a viscosity method, it may also be referred to asa “viscosity average polymerization degree.”

<Measurement of Viscosity>

For the measurement of the viscosity of the ultrafine cellulosefiber-containing slurry, the ultrafine cellulose fiber-containing slurrywas diluted to a solid concentration of 0.4% by mass, and then, theresulting slurry was homogeneously stirred using a disperser at 1500rpm. The obtained slurry was left at rest for 24 hours, and thereafter,the viscosity of the slurry was measured using a type B viscometer(manufactured by BROOKFIELD; analog viscometer T-LVT). Regardingmeasurement conditions, the viscosity obtained when the slurry wasrotated at 25° C. at 3 rpm for 3 minutes was measured.

<Measurement of Haze>

Haze is a scale for the transparency of the ultrafine cellulosefiber-containing slurry. The lower the haze value, the higher thetransparency of the slurry that can be obtained. For the measurement ofa haze, the ultrafine cellulose fiber-containing slurry after completionof the mechanical treatment step (fibrillation step) is diluted with ionexchange water to result in a solid concentration of 0.2% by mass, andthen, the resulting slurry is homogeneously stirred. The haze wasmeasured using a hazemeter (manufactured by MURAKAMI COLOR RESEARCHLABORATORY Co., Ltd.; HM-150). The measurement was carried out using aglass cell for liquid having an optical path length of 1 cm(manufactured by Fujiwara Scientific Co., Ltd.; MG-40; inverse opticalpath) in accordance with JIS K 7136. The zero point was measured withion exchange water which was placed in the glass cell.

<Measurement of Amount of Phosphoric Acid Groups Introduced>

The amount of phosphoric acid groups introduced was measured accordingto a conductometric titration method. Specifically, fibrillation wascarried out by a mechanical treatment step (fibrillation step), and theobtained ultrafine cellulose fiber-containing slurry was then treatedwith an ion exchange resin. Thereafter, while adding a sodium hydroxideaqueous solution to the resulting slurry, a change in the electricalconductivity was obtained, and the amount of phosphoric acid groupsintroduced was thereby measured.

In the treatment with the ion exchange resin, a strongly acidic ionexchange resin (manufactured by Organo Corporation; Amberjet 1024;conditioned) was added at a volume ratio of 1/10 to a slurry containing0.2% by mass of the ultrafine cellulose fibers, and the resultantmixture was stirred for 20 minutes. Then, the mixture was poured onto amesh having 200-μm apertures to separate the resin from the slurry. Inthe alkali titration, a change in the electric conductivity valueindicated by the dispersion was measured, while adding a 0.1 N aqueoussolution of sodium hydroxide to the ultrafine cellulose fiber-containingslurry after the ion exchange.

This conductometric titration confers a curve shown in FIG. 1 as analkali is added. First, the electrical conductivity is rapidly reduced(hereinafter, this region is referred to as a “first region”). Then, theconductivity starts rising slightly (hereinafter, this region isreferred to as a “second region”). Then, the increment of theconductivity is increased (hereinafter, this region is referred to as a“third region”). The boundary point between the second region and thethird region is defined as a point at which a change amount in the twodifferential values of conductivity, namely, an increase in theconductivity (inclination) becomes maximum. In short, three regionsappear. Among them, the amount of the alkali required for the firstregion among these regions is equal to the amount of a strongly acidicgroup in the dispersion used in the titration, and the amount of thealkali required for the second region is equal to the amount of a weaklyacidic group in the dispersion used in the titration. The amount (mmol)of the alkali required for the first region in the curve shown in FIG. 1is divided by the solid content (g) in the dispersion as a titrationtarget to obtain the amount (mmol/g) of the first dissociated alkali.The thus obtained amount is defined as the amount of phosphoric acidgroups introduced.

<Measurement of Yellowness of Ultrafine Cellulose Fiber-ContainingSheet>

In order to evaluate the yellowness of the ultrafine cellulosefiber-containing slurry, the yellowness of the ultrafine cellulosefiber-containing sheet was evaluated. A predetermined amount ofultrafine cellulose fiber-containing slurry having a solid concentrationof 0.5% by mass was fractionated, so that the absolute dry basis weightof a final sheet became 40 g/m², and it was then poured into Separotoholding a PVDF membrane filter having a pore diameter of 650 nm. Then,dehydration was carried out by performing suction filtration, so as toproduce a wet sheet containing ultrafine cellulose fibers, in which thesolid content of the ultrafine cellulose fibers was 4% by mass or more.The wet sheet containing ultrafine cellulose fibers was peeled off fromthe membrane filter, and was then placed on a polycarbonate plate.Thereafter, the wet sheet was subjected to drying under tension for 2days in a humidity conditioning chamber at 23° C. and a relativehumidity of 50%, so as to obtain an ultrafine cellulose fiber-containingsheet.

The yellowness (YI) of the ultrafine cellulose fiber-containing sheetwas measured in accordance with JIS K 7373, using Colour Cute i(manufactured by Suga Test Instruments Co., Ltd.). Subsequently, theyellowness (YIN) of the sheet relative to a film thickness of 30 μm wascalculated according to the following conversion equation:

Yellowness (YI₃₀) of sheet relative to film thickness of 30μm=yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm)).

TABLE 2 Ultrafine cellulose fiber-containing slurry Viscosity of Haze ofAmount of Ultrafine cellulose Supernatant Viscosity average 0.4 mass 0.2mass phosphoric fiber-containing sheet yield polymerization % slurry %slurry acid groups Yellowness [%] degree [mPa · s] [%] [mmol/g] YI₃₀ Ex.1 98.6 862 14040 2.2 1.15 0.40 Ex. 4 99.7 862 11400 2.8 1.14 0.42 Ex. 5100 854 13960 2.0 1.14 0.33 Ex. 7 100 — — 0.9 1.55 — Comp. 100 884 140002.1 1.19 0.59 Ex. 1 Comp. 99.2 — — 1.4 1.60 — Ex. 2

The pulp comprising phosphorylated cellulose fibers obtained in each ofthe Examples had high ISO whiteness and a low b* value. Even in a casewhere such a pulp comprising phosphorylated cellulose fibers having highwhiteness and a low b* value was fibrillated, the physical properties ofa slurry comprising ultrafine cellulose fibers were maintained, andalso, the yellowness of a sheet formed from the ultrafine cellulosefiber-containing slurry was suppressed at a low level.

Besides, in the above description, the pulp comprising cellulose fibersobtained in Comparative Example 1 corresponds to a pulp comprisingcellulose fibers before being subjected to the bleaching step. As such,by comparing Example 1 with Comparative Example 1, a difference betweenthe amount of phosphoric acid groups in cellulose fibers before thebleaching step and the amount of phosphoric acid groups in cellulosefibers after the bleaching step could be calculated. The difference wasfound to be 0.04 mmol/g.

In addition, a difference between the viscosity average polymerizationdegree of ultrafine cellulose fibers obtained by fibrillation of a pulpcomprising cellulose fibers before the bleaching step and the viscosityaverage polymerization degree of ultrafine cellulose fibers obtained byfibrillation of a pulp comprising cellulose fibers after the bleachingstep was found to be 22.

Cellulose fibers before the bleaching step was subjected to afibrillation treatment to obtain an ultrafine cellulose fiber-containingslurry 1 having a concentration of 0.4% by mass, and the viscosity ofthe slurry 1 was defined as P. Cellulose fibers after the bleaching stepwas subjected to a fibrillation treatment to obtain an ultrafinecellulose fiber-containing slurry 2 having a concentration of 0.4% bymass, and the viscosity of the slurry 2 was defined as Q. The P/Q valuewas found to be 1.0.

1. A pulp comprising cellulose fibers having 0.5 mmol/g or more ofphosphoric acid groups or phosphoric acid group-derived substituents,wherein when the pulp is processed into a sheet under the followingcondition A and four sheets are then laminated on one another, the ISOwhiteness of the obtained laminate measured in accordance with JIS P8148, with the exception that the test piece defined by JIS P 8148 isset to be the obtained laminate, is 82% or more: (Condition A) ionexchange water is added to the pulp to prepare a suspension in which theconcentration of the pulp comprising phosphorylated cellulose fibers is0.3% by mass, and then, a wet sheet having an absolute dry basis weightof 200 g/m² is formed from the suspension; the wet sheet is peeled offfrom a filter, is then placed on a stainless steel tray, and is thendried under conditions of 23° C. and a relative humidity of 50% for 3days; and both surfaces of the dried sheet are sandwiched by papers andmetal plates, and the sheet is then pressed by a pressure of 7.7 MPa for1 minute to obtain a pulp sheet.
 2. The pulp according to claim 1,wherein when the pulp is processed into a sheet under the condition Aand four sheets are then laminated on one another, the b* value of theobtained laminate according to the L*a*b* color system is 5.5 or less.3. The pulp according to claim 1, wherein when a fibrillation treatmentand a centrifugation treatment are carried out under the followingcondition a, a supernatant yield calculated according to the followingEquation b is 50% or more: (Condition a) the pulp is diluted with ionexchange water to a concentration of 0.5% by mass to obtain a slurry,and the slurry is then subjected to a fibrillation treatment usingCLEARMIX-2.2S manufactured by M Technique Co., Ltd., at a rotationnumber of 21500 rpm for 30 minutes; and thereafter, the obtained slurryis diluted with ion exchange water to a solid concentration of 0.1% bymass, and the resulting slurry is then subjected to a centrifugationtreatment at 12000 G for 10 minutes,Supernatant yield (%)=solid concentration (% by mass) in supernatantobtained after centrifugation treatment/solid concentration (% by mass)in slurry before centrifugation treatment×100.   (b)
 4. A slurrycomprising phosphorylated ultrafine cellulose fibers, which have 0.5mmol/g or more of phosphoric acid groups or phosphoric acidgroup-derived substituents and have a fiber width of 1000 nm or less,wherein when a sheet is formed using the slurry under the followingcondition c, the yellowness (YI₃₀) of the sheet relative to a filmthickness of 30 μm calculated according to the following Equation d is0.57 or less: (Condition c) the concentration of the phosphorylatedultrafine cellulose fibers in the slurry is adjusted to be 0.5% by mass,and the slurry is then dehydrated by suction filtration using, as afilter medium, a PVDF membrane filter having a pore diameter of 650 nm,until the solid content in the phosphorylated ultrafine cellulose fibersbecomes 4% by mass or more; and thereafter, the resulting slurry issubjected to drying under tension for 2 days in a humidity conditioningchamber at 23° C. and a relative humidity of 50%, so as to obtain asheet having an absolute dry basis weight of 40 g/m²,Yellowness (YI₃₀) of sheet relative to film thickness of 30 μm=yellowness (YI) of sheet×(30 (μm))/(film thickness of sheet (μm)),  (d) wherein, in the above equation, the yellowness (YI) of a sheet ismeasured in accordance with JIS K 7373.